Modern internal combustion engines generate approximately 20% of all of their hydrocarbon emissions by evaporative means, and as a result, automobile fuel vapor emissions to the atmosphere are tightly regulated. For the purpose of preventing fuel vapor from escaping to the atmosphere an Evaporative Emissions Control (EVAP) system is typically implemented to store and subsequently dispose of fuel vapor emissions. The EVAP system is designed to collect vapors produced inside an engine's fuel system and then send them through an engine's intake manifold into its combustion chamber to get burned up as part of the aggregate fuel-air charge. When pressure inside a vehicle's fuel tank reaches a predetermined level as a result of evaporation, the EVAP system transfers the vapors to a charcoal, or purge canister. Subsequently, when engine operating conditions are conductive, a purge valve opens and vacuum from the intake manifold draws the vapor to the engine's combustion chamber. Thereafter, the purge canister is regenerated with newly formed fuel vapor, and the cycle continues.
As opposed to vacuum in naturally aspirated applications, at higher throttle levels a turbocharged/supercharged engine's intake manifold can see relatively high boost pressures generated by forced induction. A purge valve, which is not designed to withstand high boost pressures, can sometimes be damaged under such conditions. Damage to the purge valve, in tun, is sufficient to incapacitate an EVAP system. Typically, a simple check-valve is employed in a purge harness of an engine with forced induction to prevent high boost pressures from impacting the purge valve.
In addition to fuel vapor recovery function, an EVAP system is required to perform a leak-detection function. To that end, a known analog leak-detection scheme employs an evaporative system integrity monitor (ESIM) switch which stays on if the system is properly sealed, and toggles off when a system leak is detected. When the ESIM switch is toggled off, an engine control unit (ECU) detects the change and alters an operator of the vehicle with a malfunction indicator.
Furthermore, an EVAP system's ability to detect leaks must be regularly verified in engine key-off mode via a so-called rationality test. The rationality test confirms the ESIM switch functionality through a simulated system leak which is generated by opening the purge valve to relieve a low level of system vacuum (approximately 0.5 KPa) retained from when the engine was running. An ECU then looks for the ESIM switch to toggle from on to off, which is an indicator that the switch is functioning correctly. For the rationality test to be performed in a forced induction engine, however, a leak-detection scheme utilizing an ESIM switch requires a two-way low airflow communication between the purge valve and the intake manifold. A simple check-valve does not permit two-way flow, therefor it will not support both purge valve over-pressure protection and ESIM functions in an EVAP system of a forced induction engine.
In view of the above, an effective apparatus is needed for permitting an EVAP system to accomplish its prescribed fuel evaporative emissions purge and leak detection functions in forced induction applications, while also protecting the system components from damage that can result from high boost pressures.